Detecting and imaging protein-protein interactions during G protein-mediated signal transduction in vivo and in situ...

An important goal in cell biology has been to observe dynamic interactions between protein molecules within a living cell as they execute the reactions of a particular biochemical pathway. An important step toward achieving this goal has been the development of noninvasive fluorescence-based detection and imaging techniques for determining whether and when specific biomolecules in a cell become associated with one another. Furthermore, these techniques, which take advantage of phenomena known as bioluminescence- and fluorescence resonance energy transfer (BRET and FRET, respectively) as well as bimolecular fluorescence complementation (BiFC), can provide information about where and when protein-protein interactions occur in the cell. Increasingly BRET, FRET, and BiFC are being used to probe interactions between components involved in G protein- mediated signal transduction. Heptahelical (7TM) receptors, heterotrimeric guanine nucleotide binding proteins (G proteins) and their proximal downstream effectors constitute the core components of these ubiquitous signaling pathways. Signal transduction is initiated by the binding of agonist to heptahelical (7TM) receptors that in turn activate their cognate G proteins. The activated G protein subsequently regulates the activity of specific effectors. 7TM receptors, G proteins, and effectors are all membrane-associated proteins, and for decades two opposing hypotheses have vied for acceptance. The predominant hypothesis has been that these proteins move about independently of one another in membranes and that signal transduction occurs when they encounter each other as the result of random collisions. The contending hypothesis is that signaling is propagated by organized complexes of these proteins. Until recently, the data supporting these hypotheses came from studying signaling proteins in solution, in isolated membranes, or in fixed cells. Although the former hypothesis has been favored, recent studies using BRET and FRET have generally supported the latter hypothesis as being the most likely scenario operating in living cells. In addition to the core components, there are many other proteins involved in G protein signaling, and BRET and FRET studies have been used to investigate their interactions as well. This review describes various BRET, FRET, and BiFC techniques, how they have been or can be applied to the study of G protein signaling, what caveats are involved in interpreting the results, and what has been learned about G protein signaling from the published studies.